I've been absolutely loving my new Canon 5D Mark II. It's a serious camera and having a full-frame sensor means I'm shooting a lot with my Canon 70-200mm f/2.8L IS USM lens. Together though, that combination weighs over 5 pounds and I rarely want to put that around my neck. I tend to sling the camera over my shoulder instead which actually works quiet well, with one really big problem.
The dial pictured above is the mode dial. It is one of the most important controls and pretty much changes everything about the cameras behavior. I tend to shoot mostly in Av mode when I'm out and about. When I sling the camera over my shoulder with the 70-200mm lens I carry it "upside down" so the lens hangs in a natural way. Unfortunately, that means this knob will rest against my side, and as I walk it can move into a different mode!
You then lift the camera and shoot only to realize you are in some crazy mode and the pictures are horrible!
If there was ever a control that could use a small, flip-lock this is it. It needs a safety.